1. Field of the Invention
This invention relates to a shaft support assembly for use in a polygon mirror drive motor for laser beam printers or a motor for magnetic disk equipment, VTR and the like. Laser polygon mirror drive motor assemblies present a singular set of shaft support conditions. The high speed motor must exhibit a high rotational accuracy with a minimum of shaft whirling; this is particularly true in modern laser printers where high speed and highly minute images and very dense memory are involved. The shaft support conditions are, however, quite stable. The rotor reaches its high operating speed almost instantaneously and operates at a constant speed.
2. Description of the Prior Art
It is known that problems relating to rotational accuracy and to contamination of the overall machine are directly related to performance of the shaft support assembly used to support the high speed rotor. The speed and accuracy of rotation of conventional rolling element bearings are limited by the accuracy of the working rolling members and inner and outer races. The high rotational speeds also dramatically reduce rolling element life. Consequently, rolling element bearings are not suitable for such applications. Instead, fluid-lubricated plain bearings have been employed in shaft support assemblies for laser polygon mirror drive motors. Such bearings are known to be effective in applications requiring high speed rotation and "high accuracy rotation". Of course, various improvements must be made in such plain bearing shaft support assemblies for use in mirror drive motors of the above type.
In a plain bearing shaft support assembly, a fluid lubricant film is formed between sliding surfaces upon rotation of the shaft or rotor. Unlike a gas bearing, a plain bearing is not limited to low load applications. Indeed, if a lubricating oil is used, an oil film of high rigidity which attains "high accuracy rotation" and high load capacity can be obtained. Accordingly, such plain bearings can be designed to be of a shorter length as compared with a gas bearing, thus enabling a realization of a compact motor.
There are, however, certain problems presented by the use of plain bearings in shaft support assemblies in polygon mirror drive motors. For example, in a plain bearing making use of a lubricating oil, oil leakage is always a problem, and dispersion of oil during high speed rotation poses a problem in the practical use of polygon mirror drive motors. Another problem is that plain bearings typically provide thrust or radial support, but not both. Finally, in applications demanding accurate shaft positioning the clearances in plain bearings must be accurately maintained. Manufacturing to the close tolerances required for such applications is expensive--if it is possible at all. Moreover, adjustment is particularly difficult since adjustment of one clearance can affect another clearance.
To cope with the problem of oil leakage, a magnetic fluid bearing has been proposed for use in polygon mirror drive motors. Such bearings include a permanent magnet and a magnetic fluid. The magnetic fluid serves two functions: sealing and providing lubrication. The magnetic fluid can be formed by treating magnetic powders with a surface active agent and dispersing the same in a base oil.
As noted in U.S. Pat. No. 4,938,611 to Nii et al. there are two basic types of magnetic fluid bearings. One type of magnetic fluid bearing retains a magnetic fluid on sliding bearing surfaces by magnetizing the same by a cylindrical-shaped permanent magnet. The other type of magnetic fluid bearing has a permanent magnet arranged at an end of the bearing and the permanent magnet and a permeable rotating shaft constitute a magnetic fluid sealing to have a magnetic fluid filled in a bearing section for lubrication. These two types of magnetic fluid bearings are intended for the prevention of dispersion of a magnetic fluid by magnetizing and providing the same with a sealing function.
There are, however, problems associated with such bearing constructions. These problems are discussed in detail in the aforementioned U.S. Pat. No. 4,938,611 to Nii et al.
According to Nii et al., these problems can be addressed by providing a magnetic fluid sealing which constitutes a bearing apparatus spaced away from a bearing section to provide therebetween a space which accommodates a cubical expansion of a lubricant and maintains a constant amount of a magnetic fluid in a magnetic fluid sealing section to prevent dispersion of the magnetic fluid upon rotation of high speeds. In addition, a mechanism for circulating the magnetic fluid through making use of shaft rotation is provided to prevent deterioration of performance due to temperature rise accompanied by viscous shearing of the magnetic fluid.
The shaft support assembly of Nii et al. includes a housing formed of a non-magnetizable material and having a bottom portion, a fluid lubricating type radial bearing means coaxially located in the housing and having a magnetic fluid sealing section and a radial bearing section, with a thrust bearing means provided at the bottom portion of said housing. A rotating shaft of a permeable material is rotatably supported by radial bearing means and thrust bearing means to extend through the radial bearing means, and the magnetic surface of fluid lubricant filled in the radial bearing section.
While the shaft support assembly disclosed by Nii et al. might solve the oil leakage problem experienced with previous polygon mirror motor support bearings, it fails to address the other two problems mentioned above--the need for close tolerances to permit precise alignment of the shaft and the need to provide simultaneously adjustable radial and thrust supports. As a result, the shaft support arrangement disclosed by Nii et al. is unnecessarily complex and expensive to construct. Thus, there remains a need for a simple reliable inexpensive easily adjustable bearing assembly for supporting a high speed shaft of the type used to support polygon mirrors.